A flue exhaust gas or a diesel engine exhaust gas includes a particulate matter (PM) such as soot, which has been a cause for air pollution. To remove the PM, a filter made of a ceramic material or the like (a diesel particulate filter: DPF) is broadly used. The ceramic DPF can be used for a long period of time, but a defect such as cracking or melting due to heat deterioration or the like occurs at times, and the PM might leak though a leak amount is small. When such a defect occurs, it is remarkably important from the viewpoint of the prevention of the air pollution to immediately detect the occurrence of the defect and to recognize the abnormality of a device.
As a method of detecting the occurrence of such a defect, there is a method of providing a particulate matter detection device on the downstream side of the DPF (e.g., see JP-A-60-123761).